Advanced Materials for Agriculture, Food, and Environmental Safety (Tiwari/Advanced) || Application of Zero-valent Iron Nanoparticles for Environmental Clean Up

Ashutosh Tiwari and Mikael Syväjärvi (eds.) Advanced Materials for Agriculture, Food,

and Environmental Safety, (385–420) 2014 © Scrivener Publishing LLC

385

14

Application of Zero-valent Iron Nanoparticles for Environmental Clean Up

Ritu Singh*,1,2 and Virendra Misra2

1Department of Environmental Science, School of Earth Sciences, Central

University of Rajasthan, Ajmer, Rajasthan, India 2Environmental Toxicology Division, CSIR-Indian Institute of Toxicology Research

(IITR), Lucknow, Uttar Pradesh, India

AbstractTh e contamination of soil and groundwater through a variety of toxic/hazardous

compounds has become a global environmental problem and is a great challenge

before the scientifi c community of the world. In response to the continuously

increasing need to address the problems of environmental contamination, several

remediation technologies/methods have been investigated. In the advancement of

the search for potential remediation technologies, nanotechnology has introduced

a new dimension to the area of environmental clean up. Nanoparticles mediated

remediation could be considered as an eff ective alternative to the current practices

of site remediation. Th eir extremely small size and high surface area to volume

ratio impart them properties which can be exploited for degradation/reduction of

hazardous/toxic wastes. Among several nanoparticles evaluated for the purpose

of contaminant degradation, zero-valent iron nanoparticles (nFe0) have received

considerable attention over the last decade as an excellent electron donor with

the potential ability for in situ remediation of large contaminated sites. In recent

years, nFe0 has been successfully applied for degradation of a wide array of con-

taminants including polyhalogenated compounds, chlorinated solvents, dyes,

inorganic anions, heavy metals, etc. Although nFe0 is eff ective against a wide array

of pollutants, still there are specifi c issues related to the reactivity and longevity

of nFe0, their transport and fate in the environment, etc., which are of concern.

Th e aim of this chapter is to provide an overall perspective of the use of nFe0 to

*Corresponding author: [email protected]; [email protected]

386 Advanced Materials for Agriculture, Food, and Environmental

decipher potential issues related to the treatment of contaminated environmental

matrices (ground water, surface water, aquifers and soil).

Keywords: Zero-valent iron nanoparticles, stabilized iron nanoparticles, bimetal-

lic iron nanoparticles, contaminants, remediation/degradation

14.1 Introduction

Nanotechnology is an area which touches almost every aspect of the mod-ern world ranging from research application, medical fi eld, and information technology to consumer goods. In the past two decades, nanotechnology has advanced on all fronts: pharmaceutics, engineering, electronics, optics, etc. Here the question arises; why is nanotechnology gaining so much attention? Th e answer to this question resides in the unique features of nanoparticles, which are associated with their extremely small size, high surface area to volume ratio and high intrinsic energies. Th ese proper-ties signifi cantly increase the proportion of atoms located at the surface of nanoparticles in comparison to the bulk, which in turn is responsible for the enhanced tendency to interact with atoms, ions and molecules or the complexes present in their ambience. Th e magnifi cence of nanoparticles is that the novel phenomenon and properties (optical, magnetic, electric, physical, etc.) which arise due to their nanometer length scale could be exploited for a wide array of applications in domestic as well as industrial processes. For instance, Copper (Cu) nanoparticles smaller than 50 nm, are considered as super hard materials that exhibit malleability and ductil-ity completely diff erent from bulk copper. Another example is Silver (Ag), which is considered chemically inert at macroscale, whereas nanosized Ag is used as antimicrobial agent. Likewise, Gold (Au) nanoparticles display remarkably diff erent properties (color, melting point, etc.) when compared with bulk material of Au. Similar to these, numerous examples can be found in literature which show the utility of controlling and/or manipulat-ing the material at nanometer-length scale.

With the continuous advancement of industrialization and urbaniza-tion, the level of pollution has increased many folds in the past few decades, contaminating almost every compartment of the environment. Th is is aff ecting the health of millions of people worldwide and incessantly degrad-ing the environmental quality. In response to the continuously increasing need to address the problem of environmental contamination, several remediation technologies/methods have been proposed and investigated in the past. In the advancement of the search for potential remediation technologies, nanotechnology has introduced a new dimension to the area

Application of Zero-valent Iron Nanoparticles 387

of environmental clean up. In the past two decades, a nanoparticles-based remediation approach has gained signifi cant achievements in the fi eld of environmental clean up, suggesting its potential as a good alternative to the current practices of site remediation. As compared to conventional treat-ment technologies such as pump and treat, soil fl ushing, incineration, etc., nanoparticles off er an eff ective, economic and time-effi cient technology. Th eir extremely small size and excellently high surface area make them an effi cient remediation tool for both in situ and ex situ application. Among several of the nanoparticles, zero-valent metal nanoparticles show good effi ciency for reducing organic and inorganic contaminants. For instance, Zn0, Cu0, Fe0, Mg0, Pd0, etc., have been reported for the removal of various contaminants from soil, sediments and groundwater [1–3]. Being inexpen-sive and environmentally benign, zero-valent iron (Fe0) is the most widely studied nanoparticle for the treatment of environmental contaminants. Th e reducing ability of Fe0 was fi rst demonstrated in 1994 by Gillham and O’Hannesin [4] while studying the reduction of chlorinated compounds. Since then, a huge eff ort has been made to test the potential of Fe0 for reducing toxic/hazardous substances such as chlorinated hydrocarbons, chlorinated solvents, pesticides, dyes, heavy metals, etc., as indicated by the number of related studies published aft er 1994 [5–10].

With the introduction of nanotechnology in the area of environmental clean up, attempts have been made to explore the potential of nanoscale zero-valent iron (hereaft er referred to as nFe0) for treatment of environ-mental toxicants. Th e combination of nanosize eff ect and excellent reduc-ing capabilities of Fe0 has resulted in the emergence of a very powerful remediation tool for reducing the burden of toxic/hazardous wastes from the environment. Zero-valent iron has been successfully applied as reduc-tant to degrade chlorinated aliphatic and alicyclic compounds, chlorinated solvents, nitroaromatic compounds, textile dyes, chlorinated pesticides, heavy metals, etc. [11–15]. Two potential advantage of nFe0 over their micro or macro counterparts is that nFe0 can be injected deep into groundwater and aquifers to treat contaminated plume and that the reaction kinetics of contaminant degradation is much higher. Th e appealing traits of this tech-nology have led the scientifi c and engineering community to rapidly adopt it as a new alternative tool for remediation. Th e performance of nFe0 has been displayed by several investigators in batch-scale studies. Numerous fi eld-scale demonstrations have also been tested and completed in the past few years.

Apart from their fruitful applications of nFe0, there is also a wide debate among the scientifi c community, government bodies and public, regarding its toxicity, fate and behavior in the environment and its potential impact


on the ecosystem. Th is chapter gives an overview of existing applications of nFe0 in facilitating waste management and reducing the environmental burden of hazardous/toxic substances, factors governing nFe0 reactivity, practices adopted for improving the performance of nFe0, recent progress made in laboratory studies and fi eld-scale studies. In addition, various concerns related with the application of nFe0 have also been discussed at the end.

14.2 Zero-Valent Iron Nanoparticles: A Versatile Tool for Environmental Clean Up

14.2.1 Iron Chemistry

Iron is the fourth most abundant element in the earth’s crust which pri-marily exists in two valence states—one is water-soluble ferrous iron (Fe2+) and the other is water-insoluble ferric iron (Fe3+). Th e zero-valent or ele-mental state of iron is a highly reactive species and is rarely formed on the earth’s surface. In the environment, Fe0 undergoes rapid oxidation/corro-sion owing to its high unstability. Th is occurs through an electrochemical process, whereby Fe0 dissolves at anode and reduction of redox amenable species take place at cathode. Under aerobic conditions, usually dissolved oxygen is the primary electron acceptor (Eq. 14.1), whereas water predom-inately accepts electron under anaerobic conditions (Eq. 14.2).

2Fe0(s)

+ 4H+(aq)

+ O2(aq)

2Fe2+ + 2H2O

(l) (14.1)

2Fe0(s)

+ 2H2O

(l) 2Fe2+ + H

2(g) + 2OH-

(aq) (14.2)

Th e ferrous ions formed during corrosion undergo further oxidation to form ferric ions (Eqs. 14.3 and 14.4);

2Fe2+(s)

+ 2H+(aq)

+ 1/2O2(aq)

2Fe3+ + H2O

(l) (14.3)

2Fe2+(s)

+ 2H2O

(l) 2Fe3+ + H

2(g) + 2OH-

(aq) (14.4)

As evident from the above results, nFe0 mediated redox reactions pro-duce a signifi cant increase in solution pH and a concomitant decline in solution potential (Eh). In other words, introduction of nFe0 leads to development of highly reducing conditions in the system. Sun et al. [16] reported an increase from pH ~6 to 8–9 in nFe0 containing distilled water.


14.2.2 Synthesis

In general, the synthesis of nanoparticles can be grouped in two categories: one is the top-down approach and the other is the bottom-up approach. Th e top-down approach starts with large-size (i.e., granular or microscale) materials with the generation of nanoparticles by mechanical and/or chem-ical steps including milling, etching, and/or machining. Th is approach is usually not very well suited to prepare uniform nanoparticles; especially, problems are encountered in cases where required nanoparticles dimen-sions are very small. Th e bottom-up approach entails the “growth” of nano-structures atom-by-atom or molecule-by-molecule via chemical synthesis, self-assembling, and positional assembling. Th is approach illustrates the possibility of creating exact materials that are designed to have exactly the desired properties.

Numerous methods based on these two approaches are available in lit-erature for the synthesis of nFe0 such as vaccum sputtering, sonochemical method, spray pyrolysis, laser ablation, electrochemical method, gas-phase reduction, liquid-phase reduction, etc. [12, 17–20]. Among these meth-ods, gas-phase reduction and liquid-phase reduction are the most com-monly used methods. Several workers have synthesized nFe0 by reducing goethite and hematite particles with hydrogen gas at elevated temperatures (200–600°C), by decomposition of iron pentacarbonyl [Fe(CO

5)] in organic

solvents or in argon, by electrodeposition of ferrous salts, etc. [19, 21–23]. Th e generation of nFe0 by the “bottom-up” reduction of ferric (Fe3+) or

ferrous (Fe2+) salts with sodium borohydride has gained much popular-ity because of its relative simplicity with the need of only two common reagents and no need for any special equipment/instrument. In a typical preparation, 1:1 volume ratio of NaBH

4 and FeCl

3 or FeSO

4 was vigorously

mixed in the fl ask reactor (Eqs. 14.5 and 14.6).

4Fe+3 + 3BH4

- + 9H2 O

4Fe0 + 3H

2BO

3- + 12 H+ + 6H

2

(14.5)

2Fe+2 + BH4

- + 3H2 O

2Fe0 + H

2BO

3- + 4 H+ + 2H

2 (14.6)

To ensure effi cient use of the reducing agent BH4

–1, the reactor system was operated under inert conditions. Aft er about 15–30 min, when gas evolution ceased, nFe0 was harvested with vacuum fi ltration. Maintaining a thin layer of ethanol on the top of nFe0 can help to preserve the nanopar-ticles from oxidation. Experimental parameters such as pH, reactant concentrations, stirring speed, titration rate, reaction time and external temperature infl uence the composition and surface properties of produced


nanoparticles, and hence the need to be maintained constant in the experi-ments to produce consistent samples [16, 24–26].

14.2.3 Structure

Zero-valent iron nanoparticle (nFe0) has a core shell structure, wherein core is made of metallic iron and shell consists of iron oxides/hydroxides/oxy-hydroxides. Th e metallic iron present at the center holds reducing power, whereas the oxides hydroxides coating exhibits sorption characteristics. Th us core shell structure bestows nFe0 with dual characteristics of sorption and reduction, which could be of signifi cant importance in the separation and transformation of contaminants. Th e constituent of shell, i.e., iron oxides, may have either metal-like or ligand-like coordination properties, depending on the solution chemistry. At acidic pH, iron oxides acquire positive charge and show affi nity towards anionic ligands, in contrast, at higher pH, they have negative charge and form surface complexes with cationic species [16]. On one hand the shell endows nFe0 with the properties of sorption, on the other hand it also acts as a passive fi lm, which provides a physical barrier between the metal and dissolved contaminants [27]. As the nFe0 mediated reactions are driven by oxidation of Fe0 core, sustained reduction of contaminants requires localized defects in the passive fi lm, which could facilitate charge and mass transport through it. Th ese transport processes play a signifi cant role in contaminant reduction kinetics. Th e overall reactivity of nFe0 is a function of the surface concentration of these reactive sites/localized defects [28].

It is always diffi cult to measure the exact thickness of the shell due to the high reactivity of iron, which is responsible for continuously increasing shell thickness, as the oxidation reaction proceeds. However, Martin et al. [29] attempted to determine the thickness of the shell using three methods, i.e., high resolution transmission electron microscopy (HR-TEM), high resolu-tion photoelectron microscopy (HR-XPS) and complete oxidation reaction of nFe0. Th e HR-TEM images revealed that the shell thickness predominately lies in the range of 2–4 nm. Th e HR-XPS analysis also provides approxi-mately similar thickness range with an average in the range of 2.3–2.8 nm. Th e complete oxidation of nFe0 by Cu(II) indicated a shell thickness of 3.4 nm, which is consistent with results of HR-TEM and HR-XPS.

14.2.4 Environmental Application

14.2.4.1 Zero-Valent Iron Nanoparticle

Zero-valent iron nanoparticle (nFe0) is widely used to detoxify and degrade various classes of environmental contaminants. It has diminutive size, high


density of surface reactive sites and greater intrinsic reactivity; all these properties make it a potential remediation tool for organic as well as inor-ganic contaminants. Owing to its size and surface characteristics, nFe0 can be injected or pumped straight into subsurface aquifers and groundwater to treat contaminated plumes. Not only does nFe0 reduce the total content of contaminants, but it also lowers the amount of mobile and bioavailable fractions of contaminant. In addition, it also provides enormous fl exibility for both in situ and ex situ applications.

14.2.4.1.1 Organic ContaminantsZero-valent iron nanoparticle can reduce a number of halogenated hydro-carbons to benign products such as hydrocarbons, chloride and water. Wang and Zhang [26] reported complete dechlorination of 20 mg/L tri-chloroethylene (TCE) to hydrocarbons including ethene, ethane, propene, propane, butene, butane and pentane in the presence of 1.0 g nFe0. Similarly, Zhang [30] studied the dechlorination reaction of trichloroethane, trichlo-roethene and tetrachloroethene by nFe0 and found a 99% reduction within 24 hours with ethane as the major product. Other chlorinated compounds which are reported to undergo partial or complete dechlorination include tetrachloromethane, trichloromethane [31], polychlorinated biphenyls [32, 33], etc. Regarding the dechlorination pathway of chlorinated eth-anes in a nFe0 system, Song and Caraway [34] proposed that reductive β-elimination will be the major pathway for chlorinated ethanes pos-sessing α,β-pairs of chlorine atoms whereas reductive α-elimination and hydrogenolysis will be concurrently followed by compounds having chlo-rine substitution on one carbon only. Zero-valent iron nanoparticle has also been proven as an effi cient reductant for chlorinated pesticides such as Lindane and Atrazine [35–40]. Organophosphate compounds such as chlorpyrifos, tributyl phosphate have also been reported to degrade in the presence of nFe0 [41, 42].

As a strong reductant, nFe0 degrades organochlorine compounds fol-lowing Eq. 7. Th e Fe2+ ion formed during iron oxidation reaction is a good electron donor and contributes in reducing the chlorinated compounds (Eq. 14.8).

Fe0 + R-Cl + H+ Fe2+ + R-H + Cl- (14.7)

Fe2+ + R-Cl + H2O 2Fe3+ + R-H + OH- + Cl- (14.8)

Other organic contaminants eff ectively degraded by nFe0 include: nitroamines [43], nitroaromatic compounds [44], azo dyes [45, 46], poly-brominated diphenyl ethers [47], polycyclic aromatic hydrocarbon [48], etc.


14.2.4.1.2 Inorganic ContaminantsNot only can nFe0 reduce organic contaminants, but also the inorganic contaminants such as perchlorate, chromate, nitrate, arsenic [49, 50–53], etc. Th e ability of nFe0 to reduce redox-sensitive elements has been demon-strated at both bench-scale and fi eld-scale tests [11, 14, 15]. Th e degrada-tion mechanism is based on the transformation of the toxic contaminant to nontoxic or less toxic form. For example, nFe0 transforms highly solu-ble, highly mobile, extremely toxic Cr(VI) into relatively less soluble, less mobile and less toxic Cr(VI) [20, 54]. Arsenic is another example, whose +3 and +5 oxidation states are both reported to be eff ectively removed from groundwater by nFe0 [53, 55]. Also, nFe0 has been proven eff ective for degradation of alkaline earth metals such as barium, transition metals – copper, silver, lead, etc., and radioactive elements such as uranium and technetium [56–59]. Table 14.1 lists the organic and inorganic contami-nants which are reported to be successfully degraded or transformed into less toxic or nontoxic entities by nFe0.

As signifi cant variations exist in the contaminant chemistry, there are numerous possible pathways for contaminant removal in nFe0 mediated reactions such as sorption, complexation, precipitation/co-precipitation and surface-mediated chemical reduction. Generally the contaminant is removed via a combination of two or more processes. For example, the removal mechanism of Cr(VI) involve instantaneous adsorption of Cr(VI) on nFe0 surface where electron transfer takes place, and Cr(VI) is reduced to Cr(III) with oxidation of Fe0 to Fe(III). Subsequently, Cr(III) precipitates as Cr(III) hydroxides and/or mixed Fe(III) /Cr(III)hydroxides/ oxyhydroxides as per the following Eqs. (14.9–14.11).

3Fe0 + Cr2O

7- + 7H

2O 3Fe2+ + 2Cr(OH)

3 + 8OH- (14.9)

(1-x) Fe3+

(aq) +

(x) Cr3+

(aq) + 3H

2O Cr

x Fe

1-x (OH)

3 (s) + 3H+

(aq) 14.10)

(1-x) Fe3+

(aq) +

(x) Cr3+

(aq) + 2H

2O Cr

x Fe

1-x (OOH)

(s) + 3H+

(aq) (14.11)

where x varies from 0 to 1. Mixed hydroxides of Cr(III) and Fe(III) get incorporated into the iron oxy hydroxide shell of nFe0 forming (Cr

xFe

1-x)

(OH)3 or Cr

xFe

1-xOOH at the surface; in this way Cr(III) gets stabilized/

immobilized on nanoparticle surface [20, 60]. Similarly, the removal mech-anism of As(V) and As(III) involves spontaneous adsorption and co-pre-cipitation with iron(II) and iron(III) oxides and hydroxides [55, 61]. Yan et al. [62] studied the removal mechanism of Hg(II), Zn(II) and hydrogen sulphide and reported that Hg(II) sequestrates via chemical reduction to

Application of Zero-valent Iron Nanoparticles 393T

able

14

.1

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ich

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[13

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[32

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tim

es h

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[40

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24

Dye

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[45

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(Con

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)


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Rat

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[13

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ith

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per

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[38

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[48

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tach

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CP

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CO

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Cl,

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[13

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for

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s co

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hig

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th

an m

icro

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.

[13

9]

Azo

dye

Co

ngo

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Red

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D

eco

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fo

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pse

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o fi

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ncr

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nF

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dec

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tio

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ate.

[14

0]

Ino

rgan

ic

con

tam

inan

ts

Cr(

VI)

Red

uct

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pre

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an

d

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area

no

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tim

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[60

]

As(

V)

Rem

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l b

y ad

sorp

tio

n

foll

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ed b

y re

du

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III)

nF

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s(V

) fo

rms

an i

nn

er-s

ph

ere

surf

ace

com

ple

xati

on

. HC

O3

- , H4Si

O4

0, H

2P

O4

2-

are

po

ten

tial

in

terf

erin

g ag

ents

in

As(

V)

adso

rpti

on

rea

ctio

n.

[61

]

Tab

le 1

4.1

(C

ont.

)

Application of Zero-valent Iron Nanoparticles 395C

on

tam

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Inte

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ed n

Fe0

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l sys

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in

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nit

rate

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re e

ff ec

tive

ly t

han

nF

e0 o

r ce

ll

alo

ne

[14

1]

Cr(

VI)

Red

uct

ion

fo

llo

wed

by

pre

cip

itat

ion

Cr-

Fe

hyd

roxi

de

shel

l: st

able

an

d s

ink

fo

r

Cr(

VI)

[14

2]

NO

2- ,

NO

3-

Red

uct

ion

to

N2

R

emo

val

effi

cien

cy: 6

5-8

3%

fo

r N

O2

- an

d

51

-68

% f

or

NO

3-

[14

3]

As(

V)

Rem

ova

l b

y ad

sorp

tio

n a

nd

cop

reci

pit

atio

n

Hu

mic

aci

d d

ecre

ased

As(

V)

rem

ova

l ra

te,

UV

lig

ht

pro

mo

tes

rem

ova

l effi

ci

ency

[53

]

As(

III)

Ad

sorp

tio

nA

nio

ns

(ch

lori

de,

car

bo

nat

e, n

itra

te,

ph

osp

hat

e, s

ulf

ate

and

bo

rate

),

man

gan

ese,

an

d o

rgan

ic m

atte

r in

hib

it

As(

III)

rem

ova

l.

[14

4]

Cd

(II)

Ch

emis

orp

tio

nS

orp

tio

n-

end

oth

erm

ic &

sp

on

tan

eou

s;

foll

ow

s p

seu

do

sec

on

d-o

rder

kin

etic

mo

del

[14

5]

U(V

I), C

u(I

I), C

r(V

I), M

o(V

I)A

dso

rpti

on

fo

llo

wed

by

red

uct

ion

U(V

I) r

emo

val r

ate

com

par

ativ

ely

less

in

mu

ltie

lem

ent

syst

em.

[14

6]

PO

43

-A

dso

rpti

on

Rem

ova

l an

d r

eco

very

rat

e in

crea

ses

wit

h

incr

easi

ng

pH

.

[14

7]


elemental mercury, Zn(II) undergoes sorption to iron oxide shell followed by zinc hydroxide precipitation and hydrogen sulfi de gets immobilized on nFe0 surface as disulfi de (S

22−) and monosulfi de (S2−) species. Th e removal

mechanism of metal ions is mainly dependent on the electrode potential of the metal [57]. Th e detailed explanation is given in Section 14.3.

14.2.4.1.3 Factors Aff ecting nFe0 ReactivityWhen nFe0 is introduced into the contaminated sites, it gets exposed to diff erent environmental factors. Th ese factors all together determine the overall effi cacy of nFe0 for the contaminant in the environment. Th e fol-lowing section deals with the factors which play a crucial role in determin-ing the potential effi cacy of nFe0 for remediation of contaminants present in diff erent environmental matrices.

• Size: Th e smaller particle size is accountable for greater density of reactive surface sites or surface sites of higher intrinsic reactivity. As the size of particle decreases, parti-cle dimensions approach the size of certain physical length scales, such as the electron mean-free path and electron wavelength, which in turn results in quantum eff ects. Th ese eff ects cause changes in the Fermi level and the band gap, which ultimately leads to an increase in reactivity [63].

• Specifi c surface area: It is an important factor which con-trols the physicochemical properties of nanoparticles. Th e surface area of a nanoparticle can be determined through BET gas adsorption isotherms. Alternatively, it can be calcu-lated using Eq. 14.12.

r = 3[ρ * S]-1 (14.12)

where r is the radius of the nanoparticle, ρ is the density of Fe (7,870 kg m-3) and S is the specifi c surface area. Specifi c surface area shares a direct relationship with the reactivity of nFe0. Increasing the specifi c surface area results in an increase in the fraction of iron atom present on the parti-cle surface, thereby creating a greater reductive capacity per unit of nanoparticles [64]. Several investigators have demon-strated the supremacy of nFe0 over microscale Fe0 in terms of reactivity and reaction rate constants [35, 40, 60, 64].

• Aggregation and oxidation: Generally, because of high reactivity of nFe0, it reacts rapidly with the surrounding


media such as dissolved oxygen or water, resulting in for-mation of passive layers over the surface of nFe0. Th is pas-sive layer hinders the access of target contaminants to nFe0, resulting in the loss of reactivity and degradation effi ciency. Furthermore, due to size eff ects and high surface energy, nanoparticles tend to aggregate rapidly, resulting in forma-tion of much larger clusters which may or may not refl ect the properties of their nanoconstituents. In addition, aggrega-tion also decreases the surface area, which in turn impedes the overall reactivity of the nanoparticle. To address these issues, several strategies such as use of stabilizing agent, sup-ports, immobilization/encapsulation, etc., have been devel-oped, which not only prevent aggregation but also resist oxidation, prolonging the life of reactive nFe0 [24, 25, 35, 65].

• pH: Th e pH plays a detrimental role in corrosion of iron and thus indirectly controls the degradation of contaminants. Th e lower pH is responsible for (a) acceleration of corrosion of iron, and (b) an increase in the aqueous solubility of fer-rous (Fe2+) and ferric (Fe3+) ions, yielding less iron hydroxide precipitates on the surface of nFe0 and more exposed reac-tive sites [35]. However, an extremely low pH does not favor the contaminant reduction reaction. Th is can be explained by intensive corrosion of iron at extremely acidic pH, which results in an abrupt production of hydrogen which may pro-duce a blanket of gas bubbles around the surface of nFe0, inhibiting the contact of contaminant species to nanopar-ticles; hence the overall degradation rate decreases [66]. At higher pH, Fe2+ and Fe3+ from iron surface and OH ions in the alkaline solution react to precipitate iron oxides/hydrox-ides on the surface of iron occupying the reactive sites, thus hindering the access of contaminant species to the reactive sites, reducing the reaction rate [67, 68].

• Dosage/Loading: Th e reactivity of nanoparticles is a func-tion of concentration of reactive sites. With the increase in nFe0 dosage, there is a corresponding increase in the num-ber of reactive sites. Th e greater the number of reactive sites, the greater the number of target molecules accessing the site, hence the rate of reaction is greater [20, 38, 69]. Alternatively, if the concentration of the target contaminant exceeds the number of reactive sites available for reaction, there will be a decrease in the rate of reaction.


• Reaction temperature: As per literature, the rise in tem-perature has an accelerating eff ect on the reaction process. Th is can be attributed to an increase in the mobility of target molecules with the increase in temperature, which in turn enhances the rate of reaction [35]

• Natural Organic Matter: Th e natural organic matter such as humic acid plays an important role in contaminant reduc-tion by virtue of its functional groups such as quinones [70]. Humic acids have high binding affi nity towards Fe2+ and Fe3+ [71] and also have a strong tendency for adsorption on iron oxides surfaces [20, 72]. Th e adsorption of humic acid inhib-its iron corrosion, thereby prolonging the lifetime of nano Fe0. On the other hand, adsorbed humic acid also transfers electron from inner Fe0 to Fe3+ to facilitate reduction reac-tion. In addition, humic acid complexation with dissolved iron released from corrosion can regenerate reactive Fe2+ to reduce redox amenable species [73]. In contrast to the enhancing eff ect of humic acid, Liu et al. [74] reported that humic acid competes with the contaminants for the reac-tive sites on nFe0 surface and alters the reduction potential of neighboring surface sites, thus decelerating the contaminant reduction rate.

Th ese are some of the factors which have been widely investigated in nFe0 mediated remediation. Besides these, other factors which determine the reac-tivity of nFe0 include dissolved oxygen, hardness, oxidation-reduction poten-tial, ionic strength of groundwater, aquifer’s hydraulic properties, etc. [75].

14.2.4.2 Bimetallic Iron Nanoparticles: Improving the Reactivity of nFe0

Th e urge for enhancement of reaction kinetics and improvement in metal usage has lead to the synthesis of bimetallic nanoparticles. Iron-based bimetallic nanoparticles consist of a base metal, i.e., Fe0, as the reductant, and a second metal such as Pd, Cu, Ni or Pt as the catalyst. Th ese bimetallic nanoparticles have reaction rates that are orders of magnitude higher than the corresponding monometallic nanoparticles [26, 35, 64, 76]. Catalytic hydrodechlorination is considered as the main pathway of contaminant degradation in the case of bimetallic iron nanoparticles. Studies have shown that this pathway leads to formation of a lesser amount of toxic chlorinated intermediates during reaction [31, 77]. Table 14.2 shows the

Application of Zero-valent Iron Nanoparticles 399T

able

14

.2

Co

mm

on

co

nta

min

ants

deg

rad

ed b

y b

imet

alli

c ir

on

nan

op

arti

cles

an

d s

tab

iliz

ed o

r su

pp

ort

ed n

Fe0

.

Co

nta

min

ant

cate

go

ry

Nan

op

arti

cle

Typ

eC

on

tam

inan

t

rem

ove

d

Rea

ctio

n m

ech

anis

mR

emar

ks

Ref

eren

ces

Ch

lori

nat

ed a

li-

ph

atic

, ali

cycl

ic

and

aro

mat

ic

com

po

un

ds

nF

e0/P

dT

rich

loro

eth

ene

(TC

E)

and

Po

ly-c

hlo

rin

ated

bip

hen

yl (

PC

B)

TC

E d

ecch

lori

nat

es t

o t

o

hyd

roca

rbo

ns

(Eth

ene,

Eth

ane,

Pro

pen

e, P

rop

ane,

Bu

ten

e, B

uta

ne,

Pen

tan

e)

and

PC

B t

o b

iph

enyl

Surf

ace

area

no

rmal

ized

rat

e co

n-

stan

ts w

ere

10-1

00 t

imes

hig

her

than

th

ose

of

com

mer

cial

ly a

vail

-

able

iro

n p

arti

cles

[26

]

nF

e0/N

iC

Cl 4

an

d C

HC

l 3R

edu

ctiv

e d

ech

lori

nat

ion

Rea

ctio

n r

ate

con

stan

t 2

-8 t

imes

hig

her

fo

r n

Fe0

/Ni

as c

om

par

ed

to n

Fe0

[14

8]

nF

e0/P

dP

enta

chlo

rop

hen

ol

Co

mp

lete

dec

hlo

rin

atio

n t

o

ph

eno

l vi

a te

tra-

, tri

-,

di-

& m

on

o-c

hlo

rop

hen

ol

Ult

raso

nic

atio

n i

mp

rove

s d

ech

lori

-

nat

ion

effi

cie

ncy

[14

9]

nF

e0/P

d2,

4-d

ich

loro

ph

eno

l

(2,4

-DC

P)

Cat

alyt

ic d

ech

lori

nat

ion

to

o-C

P o

r p

-CP

in

itia

lly,

then

to

ph

eno

l fi

nal

ly.

Hu

mic

aci

d i

nh

ibit

s d

ech

lori

nat

ion

[15

0]

nF

e0/P

d2

,2, 4

,5,5

-pen

ta-

chlo

ro-b

iph

enyl

Cat

alyt

ic

hyd

rod

ech

lori

nat

ion

Hig

h P

d l

oad

ing,

hig

h n

Fe0

/Pd

do

s-

age

and

sli

ghtl

y ac

idic

co

nd

itio

ns

favo

urs

dec

hlo

rin

atio

n

[15

1]

nF

e0/P

d

Tri

chlo

roet

hen

e

(TC

E)

Dec

hlo

rin

atio

n t

o c

is-1

,2-

DC

E a

nd

hyd

roca

rbo

ns

(eth

yne,

eth

ene,

eth

ane)

nF

e0 d

eact

ivat

ion

rea

ctio

n f

ol-

low

s fi

rst

ord

er k

inet

ics,

Cla

y

sed

imen

ts e

ff ec

t n

ano

par

ticl

es

viab

ilit

y

[15

2]

nF

e0/P

dL

ind

ane

Co

mp

lete

dec

hlo

rin

atio

n t

o

cycl

oh

exan

e

Neg

ligi

ble

dec

hlo

rin

atio

n o

bse

rved

by

com

mer

cial

mic

rosc

ale

Fe0

.

[15

3]

(Con

tin

ued

)


Co

nta

min

ant

cate

go

ry

Nan

op

arti

cle

Typ

eC

on

tam

inan

t

rem

ove

d

Rea

ctio

n m

ech

anis

mR

emar

ks

Ref

eren

ces

Ch

lori

nat

ed a

li-

ph

atic

, ali

cycl

ic

and

aro

mat

ic

com

po

un

ds

nF

e0/A

g T

etra

bro

mo

-

bis

ph

eno

l A

(TB

BPA

)

Red

uct

ive

deb

rom

inat

ion

to t

ri-B

BPA

, di-

BB

PA,

mo

no

-BB

PA a

nd

BPA

.

Ult

raso

nic

atio

n a

ssis

ts d

egra

dat

ion

[15

4]

nF

e0/N

iP

oly

-bro

min

ated

dip

hen

yl e

ther

s

(PB

DE

s)

Cat

alyt

ic d

ebro

min

atio

nD

ebro

min

atio

n r

ate

incr

ease

s w

ith

incr

easi

ng

amo

un

t o

f n

Fe0

/Ni,

Ni/

Fe

rati

o, a

nd

dec

reas

ing

init

ial

con

cen

trat

ion

of

PB

DE

s.

[15

5]

nF

e0/N

iA

rocl

or

12

42

Cat

alyt

ic d

ech

lori

nat

ion

to

bip

hen

yl

Hig

h n

Fe0

/Ni

do

sage

an

d h

igh

Ni

con

ten

t fa

vou

rs d

ech

lori

nat

ion

reac

tio

n.

[15

6]

Cel

lulo

se a

ceta

te

sup

po

rted

nF

e0

Tri

chlo

ro-e

thyl

ene

Dec

hlo

rin

atio

nM

emb

ran

e su

pp

ort

ed n

Fe0

sho

wed

syn

ergi

stic

eff

ect

on

dec

hlo

rin

atio

n

[65

]

CM

C s

tab

iliz

ed

nF

e0/P

d

Lin

dan

e D

ihal

oel

imin

atio

n a

nd

deh

y-

dro

hal

oge

nat

ion

un

der

anae

rob

ic c

on

dit

ion

s,

Oxi

dat

ive

deg

rad

atio

n b

y

dec

hlo

rin

atio

n /

deh

y-

dro

hal

oge

nat

ion

un

der

aero

bic

co

nd

itio

ns

Stab

iliz

ed b

imet

alli

c n

ano

par

ticl

es

mo

re e

ff ec

tive

un

der

an

aero

bic

con

dit

ion

s

[15

7]

Atr

azin

eD

ech

lori

nat

ion

fo

llo

wed

by

dea

lkyl

atio

n u

nd

er a

nae

ro-

bic

co

nd

itio

ns,

dea

lkyl

atio

n

un

der

aer

ob

ic c

on

dit

ion

s

Tab

le 1

4.2

(C

ont.

)

Application of Zero-valent Iron Nanoparticles 401C

on

tam

inan

t

cate

go

ry

Nan

op

arti

cle

Typ

eC

on

tam

inan

t

rem

ove

d

Rea

ctio

n m

ech

anis

mR

emar

ks

Ref

eren

ces

Ch

lori

nat

ed a

li-

ph

atic

, ali

cycl

ic

and

aro

mat

ic

com

po

un

ds

CM

C s

tab

iliz

ed

nF

e0/P

d

Tri

chlo

roet

hyl

ene

Co

mp

lete

dec

hlo

rin

atio

n

wit

ho

ut

form

atio

n o

f to

xic

inte

rmed

iate

s

Dec

hlo

rin

atio

n r

ate

of

CM

C-n

Fe0

/

Pd

17

tim

es f

aste

r th

at o

f n

Fe0

/

Pd

.

[24

]

CM

C-C

u/n

Fe0

1, 2

, 4-t

rich

loro

-

ben

zen

e

Seq

uen

tial

dec

hlo

rin

a-

tio

n a

nd

cat

alyt

ic

hyd

roge

nat

ion

Rat

e-d

eter

min

ing

step

is

the

elec

tro

-

ph

ilic

H*

add

itio

n t

o t

he

do

ub

le

bo

nd

of

ben

zen

e ri

ng

foll

ow

ed b

y

the

C–

Cl

scis

sio

n.

[15

8]

CM

C-P

d/n

Fe0

p-n

itro

chlo

ro-

ben

zen

e

(p-N

CB

)

Dec

hlo

rin

atio

n t

o a

ni-

lin

e w

ith

tra

ce a

mo

un

t

of

p-c

hlo

roan

ilin

e as

inte

rmed

iate

p-N

CB

to

xici

ty g

reat

ly r

edu

ces

and

bio

deg

rad

abil

ity

imp

rove

s

[86

]

CM

C-P

d/n

Fe0

2,4

-dic

hlo

rop

hen

-

oxy

acet

ic a

cid

Ad

sorp

tio

n f

oll

ow

ed b

y

red

uct

ion

to

2-c

hlo

rop

hen

-

oxy

acet

ic a

cid

an

d fi

nal

ly

to p

hen

oxy

acet

ic a

cid

Rea

ctio

n p

H a

nd

CM

C/n

Fe0

rat

io

sign

ifi c

antl

y aff

ect

s th

e re

du

ctio

n

pro

cess

[15

9]

nF

e0/C

u w

ith

acti

vate

d c

arb

on

sup

po

rt

γ-H

CH

Sim

ult

aneo

us

adso

rpti

on

an

d

dec

hlo

rin

atio

n

Tet

rach

loro

cycl

oh

exen

e an

d c

hlo

-

rob

enze

ne

iden

tifi

ed a

s m

ajo

r

inte

rmed

iate

an

d fi

nal

pro

du

ct.

[89

]

nF

e0/P

d-a

lgin

ate

Tri

chlo

ro-e

thyl

ene

(TC

E)

Co

mp

lete

dec

hlo

rin

atio

n t

o

eth

ane

and

bu

tan

e

> 9

9%

TC

E r

edu

ctio

n w

ith

in 4

ho

urs

. Les

s th

an 3

% F

e re

leas

e

fro

m s

up

po

rt.

[96

]

Ben

ton

ite

sup

-

po

rted

nF

e0

Met

hyl

ora

nge

(MO

)

Ad

sorp

tio

n f

oll

ow

ed b

y

red

uct

ive

clea

vage

of

azo

bo

nd

s

Deg

rad

atio

n s

ign

ifi c

antl

y aff

ect

ed

by

pH

, nF

e0 d

osa

ge, i

nit

ial

con

c.

of

MO

, an

d t

emp

erat

ure

.

[90

]

(Con

tin

ued

)


Co

nta

min

ant

cate

go

ry

Nan

op

arti

cle

Typ

eC

on

tam

inan

t

rem

ove

d

Rea

ctio

n m

ech

anis

mR

emar

ks

Ref

eren

ces

Ino

rgan

ic

con

tam

inan

ts

nF

e0/N

iN

O3

-R

edu

ctio

nN

ear

neu

tral

pH

fav

ou

rab

le f

or

red

uct

ion

.

[16

0]

nF

e0/A

gP

Red

uct

ion

Als

o e

xhib

its

anti

mic

rob

ial

and

anti

fun

gal

acti

viti

es

[16

1]

Res

in s

up

po

rted

nF

e0 (F

erra

gel)

Cr(

VI)

an

d P

b(I

I)R

edu

ctio

n o

f C

r(V

I) t

o

Cr(

III)

, Pb

(II)

to

Pb

(0)

Fer

rage

l red

uct

ion

rat

e 30

tim

es

hig

her

th

an t

hat

of

Fe0

po

wd

er

and

fi l

lin

gs

[64

]

CM

C s

tab

iliz

ed

nF

e0

Cr(

VI)

Red

uct

ive

imm

ob

iliz

atio

nn

Fe0

red

uce

s C

r(V

I) l

each

abil

ity

[50

]

Cal

ciu

m a

lgin

ate

entr

app

ed n

Fe0

NO

3-

Red

uct

ion

nF

e0 ca

n b

e eff

ect

ivel

y en

trap

ped

in

bea

ds

wit

ho

ut

sign

ifi c

ant

dec

reas

e

in r

eact

ivit

y to

war

ds

NO

3-

[92

]

Cal

ciu

m a

lgin

ate

entr

app

ed n

Fe0

Cr(

VI)

Red

uct

ion

R

edu

ctio

n f

oll

ow

s p

seu

do

fi r

st o

rder

kin

etic

s.

[93

]

Ben

ton

ite

sup

-

po

rted

nF

e0

Cr(

VI)

Red

uct

ion

Als

o a

ble

to

rem

ove

Pb

(II)

, Cu

(II)

and

Zn

(II)

fro

m e

lect

rop

lati

ng

was

tew

ater

. Exh

ibit

reu

sab

ilit

y.

[91

]

Pil

lare

d b

ento

nit

e

(Al-

ben

t) s

up

-

po

rted

nF

e0

Cr(

VI)

Red

uct

ion

Red

uct

ion

effi

cie

ncy

mu

ch h

igh

er

than

nF

e0 a

lon

e. n

Fe0

/Al-

ben

t

exh

ibit

go

od

sta

bil

ity

and

reu

sab

ilit

y

[16

2]

Tab

le 1

4.2

(C

ont.

)


list of bimetallic iron nanoparticles which have been reported to exhibit a high effi cacy for transformation of various chlorinated compounds, along with their predominant degradation mechanism. Generally, two methods are used for preparation of bimetallic nanoparticles. One method includes consecutive reduction of the second metal ions and subsequent deposi-tion onto fi rst metal particle, i.e., iron. Th is method leads to formation of nanoparticles with core-shell structure. Another method involves simulta-neous reduction of two metal ions, resulting in formation of alloy structure nanoparticles [78, 79].

Th e reason behind the enhanced reaction kinetics of bimetallic iron nanoparticles may be attributed to the catalytic eff ect of the second metal, which not only acts as catalyst but also enhances the surface area of nanoparticles [80]. Th e second metal also collects hydrogen gas produced during the corrosion of iron and dissociates it into atomic hydrogen, which is considered as a strong reductant for dehalogenation reactions [77]. Furthermore, the deposition of second metal creates many galvanic cells on the surface of iron, which accelerates corrosion of iron and enhances the kinetics of the redox reactions [81].

In bimetallic nanoparticles, the loading of second metal plays a cru-cial role in determining the rate of reaction. He et al. [82] found that the dechlorination effi ciency of nFe0/Pd for polychlorinated biphenyls increases as the loading of second metal (Pd in this case) increases. Singh et al. [35] also noticed a similar eff ect while studying dechlorination of γ-HCH in the presence of stabilized nFe0/Pd. Th e most likely explanation for this enhanced rate is: (a) concomitant increase in the amount of hydro-gen adsorbed on Pd surface with the increases in Pd content, which in turn promotes the rate and extent of dechlorination, and (b) higher catalyst loading increase in the total number of galvanic cells, increasing the rate of iron corrosion, thus increasing degradation rate. However, beyond cer-tain limits, the increase in Pd loading shows a declining eff ect on dechlo-rination effi ciency. Wang et al. [38] suggested that the excessive amount of Pd on nFe0 hinders the formation of hydrogen by nFe0 corrosion, thereby reducing the dechlorination rate.

An important concern associated with the use of bimetallic nanopar-ticles is the dislodgement of secondary metal with the corrosion of iron. As the secondary metals are the reactive sites, this issue is a major setback in the use of iron-based bimetallic nanoparticles for targeting contaminants. In view of this, two regenerative approaches were investigated by Zhu and Lim [83] for the recycling and reutilization of Pd/nFe0 particles. One approach utilized HCl and the other employed NaBH

4 for regeneration of

Pd. Pretreatment of aged nanoparticles with HCl served the purpose of


removing surface oxide layers, thereby re-exposing the covered Pd islets, whereas NaBH

4 was rejuvenated via reducing the iron oxide layer formed

on Pd/nFe0 surface to Fe0. Of these two methods, regeneration via HCl was found relatively more effi cient.

14.2.4.3 Stabilized and Supported Iron Nanoparticles: Improving the Mobility and Stability of nFe0

Th e activity of nFe0 is very high, thus stabilizing or supporting the particles is important to preserve their chemical nature until they can be contacted with the target contaminant. In general, two approaches are applied for sta-bilization of nanoparticles, i.e., pre-synthesis and post-synthesis. Between these two techniques, the pre-synthesis approach was reported to be more effi cient in producing nanoparticles with narrow size distribution and low sedimentation rate/long suspension time. Th e most widely used stabilizers are carboxymethyl cellulose (CMC), polyvinylpyrrolidone (PVP) and guar gum. Th ey diff er in their stabilization mechanism and thus their stabiliz-ing ability. Th e stabilization potential mainly depends on the functional group, molecular structure and molecular weight of the stabilizing agent. For instance, carboxylate group present in CMC forms strong complex with Fe2+ ions present in the solution, resulting in dispersion of the Fe2+ ions throughout the CMC network. Th ese Fe2+ ions then undergo rapid nucleation on addition of reducing agent, which is followed by growth of nuclei to a critical size. At this point, steric and electrostatic hindrance produced by negatively charged CMC molecule limits further growth of nuclei [81]. An increase in molecular weight of CMC leads to formation of well-dispersed nanoparticles with narrow size distribution. In the case of PVP, carbonyl group forms weak bonds with Fe2+, slowing the rate of nucleation, resulting in formation of relatively larger-sized nanoparticles. As PVP is a neutral molecule, growth of nanoparticles is predominately counter-checked by steric hindrance. Similar to PVP, guar gum is also neu-tral, but hydroxyl group of guar gum forms more stable complexes with Fe2+ as compared to carbonyl-Fe2+ complex, resulting in the synthesis of nanoparticles with relatively smaller-sized nanoparticles [84]. Among these three stabilizers, CMC produces the smallest nanoparticle. As far as stability is concerned, guar gum exhibits maximum stability followed by CMC and PVP. Moreover, high ionic strength (0.5 M NaCl and 3 mM CaCl

2) does not aff ect the stability of guar gum-stabilized nFe0 [85]. An

increase in molecular weight of PVP increases the nanoparticles suspen-sion stability, but it is not eff ective in the case of CMC.


He et al. [24] reported that the degradation effi ciency of CMC-stabilized Fe-Pd nanoparticles for trichloroethylene is ~17 times faster than their non-stabilized counterparts in terms of rate constant. In one more study conducted with para-nitrochlorobenzene, CMC-stabilized Fe-Pd nanopar-ticles were reported four times more effi cient than non-stabilized nFe0 [86]. Zhou et al. [87] successfully demonstrated the potential of CMC-Fe0/Pd nanoparticles for dechlorination of 2,4-dichlorophenoxyacetic acid to phenoxyacetic acid.

The long-term stability of nFe0 can also be enhanced by immobi-lizing it in a support. The immobilization of nFe0 with some support could simultaneously provide three advantages. The first one lies with the ability of supporting material to control the growth of nanopar-ticle as well as aggregation; the second one is related to the protection provided to nanoparticles against oxidation and hydrolysis in water; the third one is the preconcentration of the target contaminant around nFe0 via adsorption on the surface of supporting material, which in turn enhances the overall reactivity of nFe0. In addition, supported nanopar-ticles are more convenient in terms of their real application [64, 65]. Park et al. [88] successfully immobilized nFe0 on an ion-exchange resin sphere, which not only combats the problem of agglomeration but also reduces the amount of ammonia produced during nitrate reduction, which is otherwise a major limitation for nFe0. Zhang et al. [89] pro-posed kaolin as an ideal support for nFe0 and demonstrated its use in removing Pb(II) from aqueous solution. Cellulose acetate, activated carbon, bentonite, resin, etc., have been successfully used as supporting agent for nFe0 [64, 65, 90–92].

Th e entrapment of iron in calcium alginate beads [93, 94] or chitosan beads [95] serves the purpose of preventing oxidation and agglomera-tion without compromising the reactivity of nFe0. Krajangpan et al. [96] revealed that alginate gel cluster acts as a bridge that binds the nFe0 par-ticles together. Kim et al. [97] immobilized nFe0 in alginate bead to inves-tigate the degradation of trichloroethylene (TCE). Th e study reported that the iron released from alginate bead is <3% of the loaded iron. Liu et al. [98] utilized epichlorohydrin (ECH) as crosslinker to improve the mechanical strength of chitosan-nFe0 (CS-nFe0) beads. In comparison to CS-nFe0 beads, ECH-CS-nFe0 beads were found to possess higher ther-mal stability. Liu et al. [98] further demonstrated that ECH-CS-nFe0 beads could be regenerated and reused by washing with dilute HCl. Th e contami-nants which are successfully degraded by stabilized or supported nFe0 are displayed in Table 14.2.


14.3 Reduction Mechanisms and Pathways

In the case of reductive dechlorination of organochlorine compounds by nFe0, there are three possible mechanisms which are as follows:

i. Direct reduction at the metal surface by electron transfer from nFe0 to adsorbed contaminant molecules.

ii. Indirect reduction by ferrous ions (Fe2+). iii. Hydrogen produced as a result of iron corrosion is also capa-

ble of reductive dechlorination in the presence of a catalyst.

The direct reduction pathway mainly includes five steps: mass trans-fer of target molecule from solution to the surface of nFe0, adsorption of target molecule on the surface of nFe0 via formation of organic chemi-sorptions complex, reaction between the adsorbed target molecule and nFe0, desorption of products and diffusion of products into solution. The main reactions of direct reduction pathway include reductive α/β elimination (dihalo-elimination), hydrogenolysis (hydrogen substitu-tion of halogen), or hydrogenation (multiple bonds cleavage). Indirect reduction pathway includes reduction by ferrous ions and/or hydro-gen. Although Fe2+ is a good reducing agent, its reaction is generally quite slow and is probably not of great consequence. Hydrogen gas is an excellent reductant for dechlorination reactions, but is insufficient for reduction of a chlorinated organic compound in absence of a catalytic surface.

In bimetallic systems, the second metal (i.e., Pd, Cu, Ni, etc.) acts as a catalyst to generate active atomic hydrogen while Fe0 acts as an elec-tron donor. Th e coexistence of both e- donor and catalyst leads to higher dechlorination effi ciency. Th e catalytic reductive dechlorination of an organochlorine compound by a bimetallic iron nanoparticle follows three steps (second metal is denoted as M):

i. Th e corrosion of iron leads to production of hydrogen.ii. M and hydrogen are combined to form a transitional com-

pound M.H2 with H

2 embedded in the crystal lattice.

iii. Transitional compound dechlorinates organochlorine compound.

All these steps are depicted in Eqs. 14.13–14.18. Figure 14.1 presents a schematic diagram of the dechlorination mechanism occurring in nFe0 and


bimetallic nFe0. Th e left and right portion represent direct reduction of an organochlorine by nFe0 and catalytic reductive dechlorination respectively.

Fe0 + 2H+ Fe2+ + H2

(in acidic solution) (14.13)

Fe0 + 2H+ Fe2+ + H2 + 2OH- (in alkaline solution) (14.14)

Fe0 + RCl + H+ Fe2+ + RH + 2Cl- (direct reduction) (14.15)

M + H2

M.H2

(14.16)

M + RCl M…Cl…R (14.17)

M.H2 + M…Cl…R RH + H+ + Cl- + M (14.18)

Reduction and precipitation of metal ions by nFe0 depend on transport of the dissolved metal ions to the surface and electron transfer (ET) to the metal ion. Potential ET pathways from the surface to the sorbed ions/mol-ecules may include:

i. Direct electron transfer (DET) from nFe0 through defects such as pits or pinholes, where the oxide layer is interpreted as a simple physical barrier.

ii. Indirect electron transfer (IET) from nFe0 through the oxide layer via the oxide conduction band, impurity bands or localized bands.

iii. Electron transfer from sorbed or lattice Fe2+ surface site.

Reduction

Co-precipitation

Adsorption andReduction

AdsorptionShell (iron

oxides/hydroxides)

Core

Me-Fe-OOH

Men+

Men+ H2O

H2

Men+

Me(n-m)+

nFe0 nFe0/PdCatalytic

hydrodechlorination

Fe0 Fe2+ + 2e–

RCI

RCI Pd---Cl---R

RH + Cl–

RH + Cl–

Pd

H

Figure 14.1 Schematic diagram of mechanisms involved in contaminant degradation in

presence of nFe0 (left portion) and nFe0/Pd (right portion).


Li and Zhang [57] explained the reduction of metal cations on the basis of the standard oxidation-reduction potential of the metal ions. For metal with E

0 very close to or more negative than that of iron (-0.41 V), such as

Zn(II) and Cd(II), the removal mechanism is sorption/surface complex formation. For metals with E

0 greatly more positive than iron, for instance

Cu(II) and Hg(II), the removal mechanism is predominantly reduction. Metals with E

0 slightly more positive than that of iron, as in case of Ni(II)

and Pb(II), can be immobilized at the nanoparticle surface by both sorp-tion and reduction.

14.4 Pilot- and Field-Scale Studies

As the nFe0-based remediation approach has considerable benefi ts over conventional treatment technologies, a rapid transfer of nFe0 technol-ogy from laboratory to fi eld-scale application has been done in the last decade. Several in situ fi eld demonstrations of nFe0 technology have been undertaken and are being developed to remediate contaminated soil and groundwater. In general, there are two ways to employ nFe0 in groundwater and soil remediation: (i) through permeable reactive barriers (PRB), which contain the reactive iron, and (ii) through injection well, wherein nanopar-ticles establish a plume of reactive iron in subsurface environment.

Puls et al. [99] used Fe0 in PRB for in situ remediation of groundwater contaminated from an old chrome-plating facility located on an US Coast Guard Air Station near Elizabeth City, North Carolina, and observed chro-mate reduced to less than 0.01 mg L-1. Wilkin et al. [100] also reported Cr(VI) reduction in groundwater from 1500 μg L-1 to <1 μg L-1 in PRB installed at US Coast Guard Support Center located near Elizabeth City, North Carolina. Similar results were observed by Flury et al. [101] in PRB installed at Willisau, Switzerland, for in situ treatment of groundwater con-taminated from wood preserving industries.

Elliott and Zhang [102] studied fi eld-scale dechlorination of trichloro-ethene-contaminated groundwater through nFe0/Pd at an industrial and manufacturing facility situated in Trenton, New Jersey, USA. As per their report, ~ 890 L of nFe0/Pd suspension at concentration of 1.5 g/L on the fi rst day and 450 L of 0.75 g/L nFe0/Pd on subsequent days, was gravity fed into trichloroethene-contaminated groundwater. Th ey observed a reduction effi ciency of about 96% over a 4 week period following injec-tion. Another fi eld test was carried out at an industrial facility located in Research Triangle Park, North Carolina, wherein 1600 gallons of nFe0 slurry (1.9 g/L) were injected into the groundwater contaminated with volatile


organic carbon (VOCs), tetra-, tri- and di-chloroethene. Aft er six weeks of nanoparticle injection, over a 90% reduction was observed in concentra-tion of VOCs and the concentration of terta-, tri-, di-chloroethene were found below groundwater quality standards [103].

Quinn et al. [104] and O’Hara et al. [105] reported high dechlorina-tion effi ciencies for DNALP contaminant trichloroethylene (TCE) during a fi eld-scale demonstration of emulsifi ed nFe0 at Cape Canaveral Air Force Station, Florida, USA. A pilot-scale demonstration of bimetallic nanopar-ticles has also been carried out at Jacksonville and Lakehurst (USA) for reduction of TCE. In comparison to emulsifi ed nFe0, bimetallic nanoparti-cles were found less effi cient, possibly due to early passivation of nanopar-ticle or use of an insuffi ciently high ratio of iron and soil to generate reducing environment in the aquifer [14]. In another pilot-scale study, 150 gallons of 0.2 g/L Pd/nFe0 were injected into 50-ft deep, aquifer contami-nated with chlorinated ethenes (perchloroethylene (PCE) and trichloro-ethylene (TCE) and polychlorinated biphenyls (PCBs)). One month later, another 150 gallons 1.0 g/L Fe-Pd (CMC = 0.6 wt%, Pd/Fe = 0.1 wt%) was injected into the same aquifer at an injection pressure <5 psi. As per results, rapid degradation of PCE and TCE was observed in the fi rst week of injec-tion, however the reducing power of nFe0 was exhausted aft er 2 weeks. To counteract this limitation CMC-stabilized nFe0 was used, which not only enhanced the dechlorination, but also appeared to boost in situ biological dechlorination reactions. Th is study proposed that CMC-stabilized nFe0 facilitated the early stage abiotic degradation and at the same time acted as a source of carbon for biotic processes, thereby sustaining the biological degradation of chlorinated ethenes in subsurface [106]. Singh et al. [107] studied remediation of Cr(VI) with nFe0 in a tannery waste-contaminated site located in Rania, Kanpur, India. Th e site has been used for many years as a dumping ground for wastes generated from leather tanneries and basic chrome sulphate industries. Th ey investigated the remediation process in constructed soil windrows and reported ~99% reduction over reaction period of 50 days.

Field-scale commercial application of nFe0 has become common in the United States, but there have been only a few fi eld-scale projects carried out in Europe so far. Among them, Bornheim, Germany, was the fi rst contaminated site where nFe0 had been applied for full-scale reme-diation. Th e site was contaminated with several tons of tetrachloroeth-ene (PCE) from an industrial laundry/dry cleaner. Th e contaminant was spread over several square kilometers of area down to a depth of 20 m. Injection of one ton of stabilized nFe0 along with 2 tons of microsized nFe0, eff ectively reduced 90% of the PCE concentration initially present.


Other full-scale applications include remediation of polychlorinated eth-enes (PCE, TCE, DCE) contaminated sites located in Hořice and Písečná, Czech Republic, using nFe0 alone [108]. Yan et al. [109] have compiled several other fi eld and pilot studies which have already been done or are still in progress.

14.5 Transport of nFe0 in Environment

Over the past years, an increasing concern regarding the transport of nFe0 in diverse matrices has been noticed. Th e reason could be attributed to the poor colloidal stability of nFe0. As soon as the nanoparticles come in con-tact with the matrix, they undergo collisions with soil grains/sediments/aquifer materials and are subjected to adsorption, dispersion and retarda-tion, thereby limiting their transport through matrix [110]. In addition, their tendency towards aggregation has caused a major setback for in situ remediation applications. To achieve satisfactory remediation in subsur-face environment, it is important to improve the transport properties of nFe0. Th e mobility of iron nanoparticles can be enhanced via meeting two requirements: (i) by preventing the aggregation of nanoparticles, and (ii) by reducing the attachment of nanoparticles to matrix. In view of this, sev-eral research groups have tested various types of surface modifi ers such as surfactants, polymers, etc., to enhance the transport of nFe0 in contami-nated site. For instance, poly(acrylic acid) and anionic hydrophilic carbon form reduce aggregation among nanoparticles via electrostatic repulsion and enhance transport of nFe0 through soil- and sand-packed columns [111]. Similarly, poly(vinyl alcohol-co-vinyl acetate-co-itaconic acid) (PV3A) and triblock copolymer also increase the stability and subsurface mobility potential of nFe0 [112, 113].

He et al. [24] investigated the mobility of CMC-stabilized nFe0 and non-stabilized nFe0 in glass column packed with 2.7 mL loamy soil sand bead through column breakthrough and elution tests. In contrast to non-sta-bilized nFe0, it was found that stabilized nFe0 readily passes through the column, and is completely dispersed in the soil. Further, ~98% of stabilized nFe0 eluted with three bed volumes of deionized water indicated that reten-tion of stabilized nanoparticles in soil is irreversible. Vechhia et al. [114] investigated the transport of micro- and nanoscale Fe0 through 0.46m-long sand-packed column in the presence of xanthan gum and obtained high elution concentration (>0.94 for micro- and >0.88 for nano-Fe0 respec-tively) and recoveries (95% for micro- and 92% for nano-Fe0 respectively) for both. Cameselle et al. [115] carried out a comparison study with eight


diff erent dispersants (aluminum lactate, sodium lactate, ethyl lactate, aspar-tic acid, polyacrylic acid, 2-hydroxypropyl-β-cyclodextrin, β-cyclodextrin and methyl-cyclodextrin) and found improved stability and transport for the all over bare nFe0, with aluminum lactate as the best stabilizer demon-strating highest elution from soil column. Besides these polymers, several researchers have also used microemulsions, starch, guar gum, silica, etc. [25, 85, 116], as delivery vehicles for transport of nFe0 in porous media.

Another interesting approach towards improving the mobility of nanoparticles in soil is the application of electric fi eld, i.e., Electrokinetics. Yang et al. [117] reported that under the infl uence of an electric magnetic fi eld, polyacrylic acid-modifi ed nFe0 could be eff ectively transported hori-zontally through packed loamy sand soil (18–25 m). Pamukcu et al. [118] also showed eff ective movement of polymer-coated nFe0 from anode to cathode through clay soil. Th ey further reported in addition to enhanced transport, electric fi eld also activates nFe0. Reddy et al. [119] investigated the electrokinetic delivery of lactate modifi ed nanoscale iron particles (LM-NIP) into low permeability kaolin soil, spiked with dinitrotoluene (DNT). Th e application of constant voltage gradient (1VDC/cm) pro-duced electro-osmotic fl ow in the soil, which in combination with electro-migration enhanced the extent of transport of LM-NIP in the soil. Besides increasing the transport, enhanced degradation of the contaminant via electrokinetic remediation is an additional benefi t of this approach [120]. Some recent studies also successfully demonstrated the eff ectiveness of electrokinetics in enhancing nFe0 transport and contaminant degradation in several nFe0 permeable reactive barriers [121, 122].

14.6 Integrated Approach

Th e integrated approach exploits the reductive capabilities of nFe0- mediated abiotic degradation and biotic degradation/microbial degradation either sequentially or concurrently. Th e combination of nFe0 aided remediation and microbial degradation processes, could provide an eff ective method for degradation of most recalcitrant compounds. Generally, highly chlori-nated compounds with fi ve or more chlorine atoms such as polychlorinated biphenyls remain biorefactory to aerobic bacteria, whereas low chlorinated compounds degrade easily in the environment. Th e basic concept of an integrated system is to transform highly chlorinated non-biodegradable compounds abiotically through nFe0 to less chlorinated compounds, which can then be easily subjected to degradation by biotic communities under aerobic/anaerobic conditions.


Several studies in literature have demonstrated the applicability of integrated systems to degrade diff erent chlorinated compounds. He et al. [69] successfully transformed 2,2,´4,5,5´-pentachlorobiphenyl to 2,2´,4-trichlorobiphenyl using nFe0/Pd and then treated the end prod-uct with aerobic bacteria to further degrade the compound. Similarly, Triclosan (2,4,4´-trichloro-2´-hydroxydiphenyl ether, TCS) was also reported to get completely transformed into nontoxic products in a sequential treatment system comprised of nFe0/Pd and laccase in the presence of syringaldehyde [123]. Polybrominated diphephenyl ether was also reported to undergo degradation in a sequential treatment sys-tem involving nFe0 and Sphingomonas sp. PH-07 [124]. Sphingomonas sp. PH-07 was also reported to degrade dechlorinated product of Triclosan in a nano- bio hybrid system [125]. Th e combined use of nFe0 and auto-trophic hydrogen-bacteria was found to have a 3.2-fold higher dechlori-nation rate for trichloroethylene (TCE) in comparison to their individual eff ect showing synergistic eff ect, of combined method on overall dechlo-rination reaction [126]. A similar synergistic eff ect was also observed by Singh et al. [127] while studying γ-HCH dechlorination with stabilized nFe0/Pd and Sphingomonas sp. strain NM05 in soil system.

14.7 Challenges Ahead

14.7.1 Toxicity

Along with nanoremediation, nanoecotoxicology has also emerged as a discipline concerned with the potential risks associated with the release of nanoparticles in the environment. In recent years several articles have been devoted to the toxic eff ect of nFe0 on bacterial cells, mammalian nerve cells, bronchial epithelial cells, fi sh embryo, etc. [128–132]. In general, nanoparticles exert toxic eff ect on microbes by disrupting cell membranes, increasing membrane permeability, interrupting energy transduction, pro-ducing reactive oxygen species, lipid peroxidation, DNA damage, etc. [128, 129]. Th e toxicity of iron is based on its ability to catalyze the formation of hydroxyl radicals (OH-) from superoxide (O

2-) and hydrogen peroxide

(H2O

2). Th ese reactive species show strong biochemical activity, aff ecting

antioxidant enzymatic activities, peroxidation of membrane lipids, modi-fi cation of nucleic acids, dysfunction of cellular components, hypoxia and eventually cause cell death and tissue injury [129, 132]. Some research-ers reported that partial oxidation/aging and surface modifi cation of nFe0, could decrease its toxic properties [130, 133, 134].


Th e increasing concern over safe use of nanoparticles demands: (i) development of rapid analytical tools and methods to detect their pres-ence in diff erent environmental compartments, (ii) monitoring their pro-duction, usage, release and disposal, and (iii) adequate models for toxicity testing and risk analysis.

14.7.2 Fate and Behavior in Environment

Nanoparticles hold enormous potential to change many aspects of our existence, some for the better and some for the worse. In other words, nanoparticles have a Janus face. Th e very properties which are exploited for numerous benefi cial applications could also produce unpredictable ecotoxicological eff ects. For example, the toxic property of nanoparticles on one hand could be utilized for the purpose of water disinfection, on the other hand the same property could be harmful to the microbial commu-nity when nanoparticles enter environment. Similarly, their high catalytic property is of great signifi cance for eff ective degradation of pollutants, but the same property could induce toxic responses when taken up by a cell [13]. When nFe0 is introduced in a system (soil, sediment, groundwater, aquifer) with an aim of degrading a contaminant, it not only interacts with the contaminant but also with the biotic component of that system. As the safety of nFe0 is uncertain till date, it is important to consider its aft ermath in order to decide its fate. A proper understanding of mobil-ity, bioavailability, toxicity and persistence of nFe0 in the environment is needed for assessing its risks. Additionally, several factors such as oxida-tion and aging, interaction with common cations, ligands and complexes, surface coating with humic acids, etc., play an important role in control-ling the fate and behavior of nFe0 in the environment, and thus need to be studied in detail.

14.8 Concluding Remarks

Th is chapter presents an overall perspective of nFe0 to decipher potential issues related to its application for remediation of sites contaminated with toxic/hazardous pollutants. As evident from numerous studies, nFe0 has the potential ability to degrade or transform a wide array of contaminants to nontoxic or less toxic form through reduction, adsorption and precipitation/co-precipitation. Although nFe0 has gained signifi cant achievements in the area of environmental clean up at both bench and fi eld scales, there are still a few grey areas, economic hurdles and unforeseen ecological threats which


could limit their acceptance among the scientifi c community, regulating agencies, private and public sector. Th e potential benefi ts and environmen-tal risks associated with the use of nFe0 should be carefully weighed before the implementation of this technology in order to minimize any unintended impact on the ecosystem. Future studies should address issues concerning the mobility, reactivity, long-term fate and ecological impact of nFe0. A fun-damental understanding of behavior, interactions and overall impact of nFe0 on environment could lead to universal acceptance of this technology.

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